1. Field of the Invention
The subject invention is directed to capacitive pressure sensors, and more particularly, to high temperature capacitive static/dynamic pressure sensors, such as sensors or microphones for detecting acoustic pressure waves in a gas turbine engine, which employs a diaphragm made from a material exhibiting high thermal strength, thermal stability and oxidation resistance at elevated temperatures.
2. Description of Related Art
Capacitive pressure sensors are well known in the art, as disclosed for example in U.S. Pat. No. 6,148,674, the disclosure of which is herein incorporated by reference in its entirety. Traditionally, these devices have had limited applicability at elevated temperatures. In particular, since the capacitance of prior art pressure sensors are normally in the picoFarad (pF) range, they are susceptible to stray capacitance and other environmental conditions. This makes it difficult to develop high temperature capacitive pressure sensors for use in harsh environment applications, such as, gas turbine applications. Indeed, there are currently no commercially available capacitive pressure sensors capable of operating above 350° C.
The stray capacitance obstacle has been overcome, in part, by a combination of guard techniques and frequency modulation (FM) capacitive transmitting technology. In order to achieve accurate and reliable gauging, all stray capacitance has to be excluded in the signal pick-off circuit. This can be prevented by a technique called guarding. Guarding is accomplished by surrounding the sensing electrode area with a non-sensing conductor that is kept at the same voltage as the sensing area itself. This technique is also used to guard a tri-axial cable connecting the pressure detector to a signal conditioning circuit. As a result, there is no loss of the input signal, even though the cable may be as long as up to 10 meters.
Another factor that influences capacitive pick-off signal is the resistance of the dielectric material, which decreases as the temperature increases. This makes the direct current (DC) signal difficult to detect at high temperatures. A frequency modulation (FM) capacitive measurement system resolves this problem. Use of FM makes the system sensitive to carrier frequency variations and ignores ESD (Electro Static Discharge) or EM (Electro Magnetic) varying fields.
Moreover, in high-temperature applications, many of the prior art capacitive pressure sensors utilize all metal diaphragms. However, one of the drawbacks of a metal diaphragm in a pressure sensor or microphone is temperature hysteresis and pressure hysteresis at high temperature. A useful capacitive pressure sensor having a diaphragm made of Inconel 750 has been manufactured and tested by the present inventors. A temperature cycle test from −55° C. to 500° C. at pressure from 15 psi to 600 psi shows that the temperature hysteresis in such a device is approximately 0.76% and the pressure hysteresis is approximately 1.3% without any thermal compensation. These values are relatively large compared to a silicon diaphragm, which is usually less than 0.25%.
Still further, in high temperature applications, packaging design is critical and a robust and reliable electrical connection between the sensor chip and base (header substrate) is essential. Usually, sensor failure at high temperature is due to the degrading of the electrical connection between the contacts and the output leads. An attempt to resolve this issue is described in U.S. Pat. No. 5,955,771 to D. Kurtz et al. D. Kurtz et al. describe a leadless electrical connection technique which is applied to a piezoresistive silicon chip that is hermetically bonded and sealed to a mounting surface of a Pyrex glass wafer using conductive glass frit. As shown in FIG. 1. of the Kurtz patent, the silicon chip is electrostatically bonded to a glass wafer. The glass wafer has several through holes which expose a portion of the contacts in the silicon wafer. The device stack is then mounted on a header substrate (also made of glass) having a few pins built in. The portion of each pin extending above the mounting surface is extended into contact apertures and making electrical connection between pins and contacts by conductive glass frit. The glass frit not only establishes the electrical connection but also provides a hermetic seal for the apertures.
The Kurtz et al. approach is based on glass-Si-glass stack. In general, the maximum safe working temperature for most Pyrex glass is taken to be the strain point (515° C.). Pyrex glass usually starts to deform if maintained at temperatures around 600° C. Some special Pyrex may hold up to 700° C. It is also found that plastic deformation takes place at 600˜700° C. in silicon. Second, the thermal mismatch between Pyrex and silicon degrades the sensor accuracy at high temperature range. Third, the above design is based on piezoresistive approach. It is known that piezoresistive pressure sensors are temperature sensitive which requires complicated temperature compensation.
In contrast, capacitive pressure sensors do not need a semiconductor material to be used as sensing element. Many other high temperature materials can be selected as sensing elements. Since capacitive pressure sensors can utilize a material that is less sensitive to temperature, they are more accurate than piezoresistive sensor. By a combination of a so-called ‘guard’ technique with capacitive interface circuit, capacitive pressure sensor can provide reliable and accurate pressure sensing at extremely high temperature environment.
There is a need therefore, for a capacitive pressure sensor which is capable of operating in a high temperature environment, uses a diaphragm material which is not susceptible to hysteresis effects and has a packaging design that includes a robust and reliable electrical connection between sensor chip and base.